Work vehicles such as bulldozers are equipped with hydrostatic transmissions (HST).
FIG. 9 shows a system that controls a speed ratio of left and right crawler tracks using a hydraulic operation lever device and HST.
In other words, as shown in FIG. 9, drive shafts of constant capacity hydraulic motors 7, 8 are coupled to sprockets 15, 16 provided on left and right hand sides of a vehicle body. The left and the right sprockets 15, 16 mesh with the left and the right crawler tracks provided on the left and the right of the vehicle body respectively. Inlet and outlet ports 7a, 7b of a left hydraulic motor 7 are connected to outlet and inlet ports 3a, 3b of a variable capacity hydraulic pump 3 via an oil path 61 and an oil path 62 respectively.
In the same way, inlet and outlet ports 8a, 8b of a right hydraulic motor 8 are connected to outlet and inlet ports 4a, 4b of a variable capacity hydraulic pump 4 via an oil path 63 and an oil path 64 respectively.
Hydraulic pressure signals (pilot pressure) are output from a hydraulic operation lever device 65 to left and right swash plate drive units 105, 106 in accordance with the operation stroke, the left and the right swash plate drive units 105, 106 drive swash plates 3c, 4c of the left and the right hydraulic pumps 3, 4 in accordance with the pilot pressure, and varies the angle of tilt, in other words the capacity (cc/reb), of the left and the right hydraulic pumps 3, 4. When the ration of capacities of the left and the right hydraulic pumps 3, 4 is varied, the speed ratio of the left and the right crawler tracks is varied. In other words, since the quantity of oil discharged per revolution from the left and the right hydraulic motors 7, 8 is constant, so if the ratio of the hydraulic oil flow rate flowing into the left and the right hydraulic motors 7, 8 is determined, in other words if the capacity ratio of the left and the right hydraulic pumps 3, 4 is determined, the ratio of the rate of revolution of the left and the right hydraulic motors 7, 8, in other words the speed ratio of the left and the right crawler tracks (left and right sprockets) is uniquely determined.
FIG. 10 shows a system that controls the speed ratio of the left and the right crawler tracks using an electric operation lever device and HST.
An electrical signal is output from an electrical operation lever device 66 in accordance with the operation stroke, and is input to a controller 120, and a control electrical signal in accordance with the speed ratio is output from the controller 120 to left and right swash plate drive units 5, 6, and the swash plates 3c, 4c of the left and the right hydraulic pumps 3, 4 are driven by the left and the right swash plate drive units 5, 6, and otherwise the system is the same as in FIG. 9.
As a related art in connection with the HST, the following patent literature 1 is available.
Also, hydrostatic steering systems (HSS) are known in which the speed ratio of the left and the right crawler tracks of a work vehicle are controlled by adjusting the capacity of hydraulic pumps, in the same way as for HST.
As a related art in connection with the HSS, the following patent literature 2 is available.
Patent literature 1: Japanese Patent Application Laid-open No. 11-59212.
Patent literature 2: Japanese Patent Application Laid-open No. 2002-293261.